FIELD OF THE INVENTION
This invention relates generally to the treatment of injured, degenerated, or diseased tissue in the human spine, for example, intervertebral discs and vertebrae themselves. It further relates to the removal of damaged tissue and to the stabilization of the remaining spine by fusion to one another of at least two vertebrae adjacent or nearly adjacent to the space left by the surgical removal of tissue. More particularly, this invention relates to the implantation of devices which can be inserted to take the structural place of removed discs and vertebrae during healing while simultaneously sharing compressive load to facilitate bony fusion by bone growth between adjacent vertebrae to replace permanently the structural contribution of the removed tissue. This invention further relates to the implantation of devices which do not interfere with the natural lordosis of the spinal column. This invention further relates to implants which are radiolucent to permit more accurate diagnostic imaging follow up.
BACKGROUND OF THE INVENTION
For many years a treatment, often a treatment of last resort, for serious back problems has been spinal fusion surgery. Disc surgery, for example, typically requires removal of a portion or all of an intervertebral disc. The most common sites for such surgery, namely those locations where body weight most concentrates its load, are the lumbar discs in the L1-2, L2-3, L3-4, L4-5, and L5-S1 intervertebral spaces. In addition, other injuries and conditions, such as tumor of the spine, may require removal not only of the disc but of all or part of one or more vertebrae, creating an even greater need to replace the structural contribution of the removed tissue. Also, a number of degenerative diseases and other conditions such as scoliosis require correction of the relative orientation of vertebrae by surgery and fusion.
In current day practice, a surgeon will use one or more procedures currently known in the art to fuse remaining adjacent spinal vertebrae together in order to replace the structural contribution of the affected segment of the disc-vertebrae system. In general for spinal fusions a significant portion of the intervertebral disk is removed, and if necessary portions of vertebrae, and a stabilizing element, frequently including bone graft material, is packed in the intervertebral space. In parallel with the bone graft material, typically additional external stabilizing instrumentation and devices are applied, in one method a series of pedicle screws and conformable metal rods. The purpose of these devices, among other things, is to prevent shifting and impingement of the vertebrae on the spinal nerve column. These bone graft implants and pedicle screws and rods, however, often do not provide enough stability to restrict relative motion between the two vertebrae while the bone grows together to fuse the adjacent vertebrae.
Results from conventional methods of attempting spinal fusion have been distinctly mixed. For example, the posterior surgical approach to the spine has often been used in the past for conditions such as scoliosis, using Harrington rods and hooks to align and stabilize the spinal column. In recent years many surgeons have adopted anterior fusion because of the drawbacks of the posterior approach, the primary problem being that in the posterior approach the spine surgeon must navigate past the spinal column and its nerve structure. However, results of anterior surgery are variable and uncertain because constraining the vertebrae from this side does not address the loads put on the spine by hyperextension, such as from rocking the body in a backwards direction.
Pedicle screws and rods, always implanted posteriorly, tend to loosen either in the bone or at the screw-rod interface if fusion is not obtained. Fusion rates for posterolateral instrumented fusions range from 50% to 90%. It must be kept in mind that plain x-rays are only 65-70% accurate in determining fusion status and most studies use this inadequate method to determine fusion status, suggesting that the non-union rate may be greater than reported. It is also known that posterior pedicle screw systems do not prevent all motion anteriorly, leading to the risk of fatigue failure of the metal and screw breakage. This continued motion may also lead to persistent pain, despite solid posterior bony fusion, if the disc was the original pain generator. These well documented failures of pedicle screws have given rise to extensive litigation in the United States.
In contrast to the U.S. common practice of using either IBF devices, implanted from the anterior position, or pedicle screws, implanted posterior, in Europe, spine surgeons use both IBF devices and pedicle screws in combination to achieve stability of the spine. These procedures may be more successful in producing fusion but are far more invasive and costly and have higher morbidity for the patient.
More generally there is a great deal of variability in technique and uncertainty in outcome for the various methods now in use for spinal surgery. For example, Fraser, R. D. points out in "Interbody, Posterior and Combined Fusions," Spine, V20(24S):1675, Dec. 15, 1995, "analysis of the literature does not indicate that one form of fusion is significantly better than another for degenerative conditions of the lumbar spine." Fraser did not have the results of recent studies involving use of metal interbody cage devices. Ray, Charles D. reported the results of the original IDE study involving his Ray Threaded Fusion Cage (Ray-TFC) in Spine V22(6):667, Mar. 15, 1997. Two hundred eight patients had two year follow-up and were reported to have 96% fusion rate with only 40% excellent results and 25% fair or poor results.
There are only two published reports on the use of the BAK Threaded Interbody Fusion Cage. The first, published by Hacker, R. J., Spine V22(6):660 Mar. 15, 1997 compares posterior lumbar interbody fusion using the BAK device to anterior and posterior fusion with allograft bone. Hacker found that patient satisfaction was equivalent but overall costs were less for the BAK. Zucherman reported on the early experience with laparoscopically assisted ALIF with BAK but no outcomes data are presented on these first 17 patients. Kuslich, S. D. presented the results of the multi-center IDE study of 947 patients who had fusions using the BAK device at the 1996 annual meeting of the North American Spine Society in Vancouver. He reported a fusion rate of 90.5% and some degree of functional improvement in 93% of patients with pain eliminated or reduced in 85.6% of patients. The data so far for these threaded cages is scanty at best. It is clear that the results are better than those for posterior fusion with or without pedicle screw instrumentation but further studies are needed. Problems with threaded devices will no doubt come to light as they are used under less controlled circumstances in greater numbers of patients.
John Kostuick, M.D., Chief of Spine surgery at John's Hopkins Hospital, Baltimore, Md. (Private Communication with James Nicholson, 2nd R. Roy Camille Meeting, Paris, France, Jan. 28, 1998) vigorously disagrees that fusion can take place within a metal IBF device which shields the bone from load. Dr. Tromanhauser, one of the inventors, in a series of 30 patients implanted with BAK cages, found that at least 9 patients had continued back pain with x-rays and CT scans that were inconclusive for determining fusion. Surgical exploration of these patients has revealed continued motion and no obvious fusion. All patients were explored at least 6 months after cage implantation, a point at which most surgeons would expect fusion.
Recent unpublished research by Dr. Elsig also indicated that 60% of the cases he reviewed had to be reoperated due to failure 6-8 months after initial surgery. There is therefore recognition and belief, especially among Kostuick Fellows who adhere to the principles of Wolfe's law, that loading the bone during fusion through the implant device connecting the opposing remaining vertebrae would yield superior fusion both in strength and in duration of healing time.
It is also well established from the study of bone growth that a bone which carries load, especially compressive load, tends to grow and become stronger. Existing stabilizing implants, in particular IBF's, do not share any of the compressive load with the new bone growth, in fact possibly shielding new bone growth from load. For example, the BAK cage is promoted as being so strong that a pair of BAK cages will support the full body load. Such shielding is well known to inhibit new bone growth and healing.
The biggest limitation in any method of fusion at the present time is the nature of available devices for bridging the space left by excision of diseased or damaged tissue. In particular, interbody fusion (IBF) devices currently on the market in the United States do not provide stability in all planes of motion. There is very little evidence to support the biomechanical stability of these devices. They are generally stable in compression (forward flexion) unless the bone is osteoporotic, which condition could lead to subsidence of the device into the adjacent vertebral body with loss of disc space height. They may be much less stable in torsion and certainly less so in extension where there is no constraint to motion except by the diseased annulus fibrosus which is kept intact to provide just such constraint. It is doubtful that a degenerative annulus could provide any long term "stiffness" and would most likely exhibit the creep typically expected in such fibro-collagenous structures.
Another problem with conventional fusion devices and with IBF's in particular is difficulty in diagnostic follow-up. In assessing whether or not fusion has taken place between adjacent vertebrae and inside the IBF device, normally plain x-rays including flexion and extension views are obtained. The usual method (Cobb) of measuring motion on these x-rays has a 3 to 5 degree range of error, well beyond the motion that may be present leading to pain. It is impossible to see inside a metal IBF with plain x-rays and conclude anything about fusion status. CT scans with reformatted images are increasingly used because of these shortcomings. Newer software for CT scanners has improved the ability to "see" within cages but the metal artifacts produced by the x-rays are still significant and limit the conclusions that can be drawn. Drs. Tromanhauser and Kant have found virtually no differences in CT scans taken immediately post-op and those taken at a six month follow-up.
Accordingly, there is wide spread recognition among spine surgeons of the need for a flexible radiolucent implant device which would replace removed degenerated tissue and be firmly affixed mechanically to opposing vertebrae. Such a device would dramatically increase the probability of successful fusion because it a) would eliminate or significantly reduce relative movement of the adjacent vertebrae and the intervertebral fixation device in extension and torsion, b) would thereby reduce or eliminate the need for supplemental external fixation, c) by compressive load sharing would stimulate rapid growth of the bone elements packed within the intervertebral device by causing osteoinduction within the bone chips, thereby accelerating fusion, d) would allow confirmation that fusion had taken place using standard CT or possibly plain x-rays, and e) would have the potential to be bioabsorbable, potentially being fabricated from such materials as a D-LPLA polylactide or a remodelable type-two collagen so as to leave in the long term no foreign matter in the intervertebral space. In addition, a flexible implant device can be fabricated in whole or in part from human bone allograft material, which is sterilized and processed, automatically matching or approximately matching the elastic properties of the patient's bone. The success rate of fusion using such an approach is anticipated to exceed the success rate of the IBF devices or the external fusion devices alone and at least equal the combined success rate of the current combination IBF and posterior instrumented technique.
However, there is currently no known method of mechanically affixing an interbody implant device, such as those known in the art as "cages," to adjacent vertebrae. All present IBF devices simply jack open the intervertebral space, relying on the muscle, ligamentous, and annular structures which surround the vertebra to hold the implants in place. The annulus is always degenerative in these cases and could not possibly function in any predictable way and therefore cannot be relied upon to provide adequate motion stability.
Furthermore, prior art cages are filled with bone chips which are shielded from compressive load by the stiff metal cage, preventing natural bone ingrowth through the porous cages because the new bone growth cannot be loaded through the rigid implant. This leads to lack of fusion because the bone, according to Wolfe's law, wants to resorb due to stress shielding by the cages. In an effort to overcome this phenomenon, some manufacturers are adding bone growth factors to the cage and/or the bone graft in an attempt to "fool" the bone into fusing through the cage. However, there is no existing method of sharing compressive loads with bone growth material and new bone growth.
Lordosis, which is a pronounced forward curvature of the lumbar spine, is a factor which needs to be taken into account in designing lumbar implants. It is known in the art that preserving the natural curvature of the lumbar spine requires designing into a new device such as the current invention a modest taper approximately equivalent to the effective angularity of the removed tissue. The restoration of normal anatomy is a basic principle of all orthopedic reconstructive surgery.
Therefore there is a perceived need for a device which simultaneously and reliably attaches mechanically to the bony spinal segments on either side of the removed tissue so as to prevent relative motion in extension (tension) of the spinal segments during healing, provides spaces in which bone growth material can be placed to create or enhance fusion, and enables the new bony growth, and, in a gradually increasing manner if possible, shares the spinal compressive load with the bone growth material and the new growth so as to enhance bone growth and calcification. The needed device will in some instances require a modest taper to preserve natural lumbar spinal lordosis. It will also be extremely useful if a new device minimizes interference with or obscuring of x-ray and CT imaging of the fusing process.
Thus it is an object of the current invention to provide a stabilizing device for insertion in spaces created between vertebrae during spinal surgery. It is a further object to create an implantable device for stabilizing the spine by preventing or severely limiting relative motion between the involved vertebrae in tension (extension) and torsion loading during healing. It is a further object to provide a device which promotes growth of bone between vertebrae adjacent to the space left by the excised material by progressive sharing of the compressive load to the bone graft inserted within the device. It is yet a further object to provide mechanical stability between adjacent vertebrae while bone grows through a lumen in the implant and at the same time not diminish the natural lordosis of the lumbar spine. It is a further object of the invention to provide a device which avoids or minimizes interference with various imaging technologies. It is yet another object of this invention to be capable of being fabricated from human bone allograft material.